This application claims priority from Japanese Patent Application No. 2005-069512, filed Mar. 11, 2005, the entire subject matter of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to an image-forming device capable of forming an image with proper density at constant.
2. Description of Related Art
In conventional image-forming devices, an electrostatic latent image is formed on the surface of a photoconductor, and thereafter, a developer is applied to the surface of the photoconductor to form a developer image. The developer image is transferred to a recording medium to form an image. Since the developer deteriorates due to aging, density of an image developed with a new developer differs from density of an image developed with an aged developer. Therefore, in conventional image-forming devices, density correction is performed to adjust properly density of images to be formed.
Japanese Patent Publication No. 2532073 provides a known method of density correction. In Japanese Patent Publication No. 2532073, at first, a patch is formed on a conveyer belt which conveys recording medium for every predetermined times of image formation. Then, density of the patch formed is measured by a sensor. Corresponding to a difference between the density of the formed patch and preset target density, a correction amount is determined.
Normally, as a developer deteriorates in accordance with increase in number of times of image formation, the developer becomes charged less. Therefore, if development is carried out at an equal developing bias voltage, the amount of developer sticking on the surface of the photoconductor increases gradually, causing rising of density of formed images. In particular, this phenomenon is conspicuous when positively charged polymerized toner is used as the developer.
However, in the method described in the above patent publication, deterioration of a developer due to increase in the number of times of image formation is not considered. Therefore, even if a correction amount is once determined, the difference between density of a formed patch and preset target density increases as the developer deteriorates in accordance with increase in the number of times of image formation.